1. Field of the Invention
The present invention relates to the microwave field and more particularly to nonreciprocal devices for surface electromagnetic waves without volume waves. The invention is primarily applicable to microwave isolators having low insertion losses in the direction of wave propagation and high attenuation in the reverse direction over a broad frequency range of the order of 4 to 20 GHz.
2. Description of the Prior Art
Surface electromagnetic waves are understood to means waves which propagate in a direction perpendicular to the magnetization of an anisotropic material such as a ferrite and in modes of a noval type designated as abnormal gyromagnetic modes arising from the anisotropic properties of ferrite.
The term nonreciprocal circuit refers to a circuit whose transmission characteristics (attenuation, phase-shift) change according to the direction of wave propagation through the circuit. Known circuits of this type include transmission-line sections (coaxial lines, waveguides, striplines circuits, and so on) which contain ferrimagnetic or gyromagnetic material such as a ferrite which is subjected to a steady magnetization field. The permeability of a material of this type when subjected to external magnetization is a tensor, which means that the impedance of the medium for a wave propagated therein depends on the orientation of the magnetic field of the wave with respect to a fixed reference related to said medium. This orientation therefore changes with the direction of propagation. It is the use of this property that forms a basis for the construction of circuits known as circulators, isolators, phase-shifters and so on, which have the function in the case of isolators, for example, of transmitting waves with low attenuation in the forward direction (a few dB and sometimes lower values) and much higher attenuation in the reverse direction (higher than 20 dB).
For a detailed description of these so-called surface waves, reference can be made for example to the articles published respectively in "Cables and Transmission", No. 4, October 1973, pages 416 to 435, under the title: "Propagation in a magnetized ferrite plate, application to novel wide-band nonreciprocal devices", and in "Transactions on Magnetics" of the Institute of Electrical and Electronic Engineers - Vol. Mag. 11 - No. 5 - September 1975, page 1276.
Furthermore, nonreciprocal microwave devices have known structures which utilize the propagation of electromagnetic surface waves in a medium consisting of a gyromagnetic or ferrimagnetic material. Devices of this type have already been disclosed in published documents, among which may be cited U.S. Pat. No. 3,845,413 in respect on "Nonreciprocal surface-wave devices" as well as U.S. Pat. No. 4,152,677 and relating to "Wide-band isolators for operation at centimeter-wavelengths".
Other modes of surface propagation including parasitic modes of volume waves or surface waves can be excited at frequencies of the waveband to be transmitted and can propagate in the gyromagnetic material simultaneously with the desired nonreciprocal surface mode. The parasitic modes which are closest to the mode employed (this latter being designated as a "dynamic mode") are volume modes. The essential aim of the present invention is to provide means for reducing the proportion of energy applied to the device which undergoes a transformation to a parasitic wave, and which is derived from the energy propagated in the dynamic mode employed.
The devices constructed in accordance with the techniques described in the patents cited in the foregoing sustain parasitic volume modes excited by the electromagnetic surface mode designated as the SEW mode (surface electromagtnetic waves). Up to the present time, no method has yet been found for systematically blocking the process of excitation of these parasitic modes. The main disturbing modes are hybrid modes which are closely related to the TE modes (dominant transverse electric modes).
It is known that, in a device of this type, a flat metallic conductor or so-called core of substantially trapezoidal shape such that the sides which are not parallel to each other are curvilinear is placed between two plates of gyromagnetic material in which the surface waves propagate and two plates forming absorption loads and placed against said plates of gyromagnetic material. This device is subjected to a magnetic field H perpendicular to the plane of the core. In all devices of this type, the surface modes are guided along the bound to the surface of the flat conductor or strip-line, said surface being parallel to the field H. In the forward direction, the surface modes are consequently guided by the long rectilinear side of the trapezoidal core and transmitted to the output of the device. On the other hand, in the reverse direction, the surface modes are guided by the curved portion of the core and absorbed by the absorption loads. Volume modes also exist in the strip-line and penetrate into the absorber.